This credit is designed primarily for low-income taxpayers. In fact, for 2006, if your earned income (basically that's income from a job or self-employment) or adjusted gross income (AGI, which can include investment earnings) is over $38,348, you make too much to qualify for this credit. If you're single and don't have a child, the right to this credit disappears when earned income or AGI passes $12,120.
If you qualify, the top credit for 2006 is $4,536 if you are married and have two or more children. The credit tops out at $2,747 if you have one child. If you don't have kids, the top earned income credit for 2006 is $412.
Unlike most tax credits, which can't reduce your tax bill below $0, if an earned income credit exceeds the amount you owe, the excess will still be refunded to you. Also, you don't have to wait until you file your return to take advantage of this credit. If you expect to be eligible for it, you can get your employer to include a portion regularly in your paychecks. To get advance payment of the credit, ask your employer for a copy of Form W-5, Earned Income Credit Advance Payment Certificate. That form is filed with your employer, not the IRS.